Metal halides are compounds which have been known for a long time. For example, alkaline earth fluorides (group IIA), 4th period transition metal fluorides or rare earth fluorides (group IIIB) are known. These metal fluorides have interesting electrical, magnetic and optical properties.
Alkaline earth fluorides have low refractive indexes, which enables them to be used in an anti-reflective layer on supports with a high reflective index.
The layers obtained from these fluorides in general have low dielectric constants, are transparent and have excellent mechanical properties.
All these special properties mean that metal fluorides are compounds which are advantageous for piezoelectric, ferromagnetic or antiferromagnetic, electro-optical, pyroelectrical or non-linear optics applications.
Mixed rare earth and alkaline earth metal halides are also known as luminescent substances used for example to convert X-rays or gamma radiation into visible light.
Many publications describe the use of mixed alkaline earth halides as luminescent substances, in particular in radiographic products.
For example, European patent EP 149148 describes radiographic image recording screens which contain, in the storage layer, a mixed alkaline earth halide of general formula BaF(XY):Eu:Sr in which X and Y are halide atoms. These luminescent substances are obtained by mixing BaF.sub.2, BaCl.sub.2, BaBr.sub.2, EuF.sub.3 and SrCl in a ball mill. The mixture is then baked red hot in a bromium vapour chamber for 1 to 5 hours at a temperature of between 800.degree. and 1000.degree. C. After cooling, the product is broken up, washed and then dried. In this way the luminescent substance described above is obtained. This technique, which is difficult to implement, does not make it possible to control the stoichiometry of the final product.
It is known that thin layers of luminescent substances can be formed by chemical vapour phase deposition. Such layers are obtained with difficulty because of the differences in vapour tension and stability of each of the constituents.
A process for obtaining layers of metal fluorides was described in U.S. Pat. No. 3,475,192. Such a process consists of coating, on a substrate, a magnesium fluoride solution in a polar solvent and heating the substrate thus covered at between 100.degree. and 1000.degree. C. In this process, it is necessary, in order to obtain a film, to use a substrate which is resistant to high temperatures.
It is known that metal fluoride layers can be obtained by the decomposition of a metal fluoride precursor.
For example, in U.S. Pat. No. 4,492,721, magnesium fluoride layers are obtained by the decomposition of fluorinated organic compounds of magnesium, such as magnesium trifluoroacetate.
U.S. Pat. Nos. 5,051,278 and 5,271,956 describe a process for forming films of metal fluorides, in particular binary and ternary alkaline earth or lanthanide fluorides. This process consists of forming a coating solution containing a non-fluorinated organometallic compound, a solvent and a fluorination agent and coating this solution on a support. The film thus obtained is then heated at a temperature of approximately 500.degree. C. in order to decompose the products contained in the coating solution into pure metal fluoride. In order to obtain a uniform layer, a temperature increase is effected with a gradient of around 50.degree. C./min.
U.S. Pat. Nos. 5,208,101 and 5,268,196 describe a process for forming layers comprising alkali metal or alkaline earth fluorides using sol-gel technology. This process consists of forming a layer on a glass substrate using a coating solution containing a light metal oxide precursor, a non-aqueous solvent and water, heating the layer in order to densify the layer of light metal oxides, and exposing this densified layer at a high temperature to a gaseous current containing fluorine. The densification of the oxide layer is effected at a temperature of around 500.degree. C. and the fluorination is effected at temperatures of around 300.degree. C.
In all the known processes set out above, the metal halide layers are obtained using high temperatures, either to vaporise the metal halides or to decompose a precursor of the metal halides, or to densify the base metal oxide layer and to halogenate this same layer.
All these processes enabling layers of metal halides to be obtained have many drawbacks related to the necessity to use high temperatures. In particular, the choice of the support for the metal halide layer is very limited. In addition, at high temperature, it is very difficult to obtain homogeneous metal halide layers having controlled stoichiometry.